Functionally graded alloys are materials having continuously or stepwise changing properties such as hardness, elasticity, thermal conductivity, electrical conductivity, etc., without developing a gradient in size with mechanical working such as cutting or the like, or chemical treatment such as etching or the like. Functionally graded materials developed to date are mostly two-component composites such as SiC/C, ZrO/W, TiC/Ni, ZrO/Ni, etc. These materials have gradually changing mixing ratios.
Conventional functionally graded materials having gradually changing mixing ratios have been produced by mixing different material powders at gradually changing mixing ratios to prepare a plurality of mixed powder sheets having gradually changing mixing ratios, laminating the mixed powder sheets along the gradually changing mixing ratios, compacting them, and sintering them. For example, Japanese Laid-Open Patent No. 5-278158 discloses a functionally graded, binary metal material produced by laminating and sintering tungsten powder and molybdenum powder at a gradually changing mixing ratio.
However, functionally graded materials produced by this method cannot be rolled or drawn, and they can be formed into desired shapes only by cutting. Thus, they are not only very expensive, but they also cannot be formed into complicated shapes. Accordingly, conventional functionally graded materials are used mainly in highly expensive applications, such as spacecraft, nuclear power generators, etc. Thus, it is very desirable to develop less expensive and easily formable functionally graded materials.
Alloys having shape recovery properties and superelasticity are widely used in various applications such as guide wires, catheters, etc. To introduce a catheter into a blood vessel and place it at a desired site in the blood vessel, a guide wire for guiding the catheter is first introduced into the desired site in the blood vessel, and the catheter is guided to the desired site in the blood vessel along the guide wire. Because human blood vessels wind and branch differently depending upon the individual, guide wires having high introduction operability and torque conveyance are required to insert the guide wires without damaging the blood vessel walls.
For this purpose, a guide wire is composed of a core wire comprising a tip end potion which is made soft by reducing its diameter, and a body portion which is relatively rigid. A coating layer is formed on the core wire, the coating being made of a synthetic resin which is inert with respect to the human body, such as polyamides, thermoplastic polyurethanes, fluoroplastics, etc.
The guide wire usually comprises a coil-shaped metal wire made of a material such as carbon steel or stainless steel. However, since wires made of these materials are easily bent, superelastic metals such as Ni—Ti alloys and the like are used for the core wires of guide wires (Japanese Patent Publication No. 2-24549). However, superelastic Ni—Ti alloys lack rigidity, even though they are sufficiently soft for this use. Therefore, they are not useful for insertion into a blood vessel, sometimes making it difficult to place them at a desired location in the blood vessel.
Additionally, because Ni—Ti alloys are relatively poor with respect to cold working, they are not easily formed into thin wires suitable for use as guide wires and the like. With respect to the gradient properties from heat treatment, it is difficult to provide the guide wire with the gradient needed to control the torque conveyance of the guide wire.
The same is true of catheters made of Ni—Ti alloys. The Ni—Ti alloy catheters are not easily inserted into a blood vessel. Also, Ni—Ti alloys cannot be easily formed into thin wires or pipes. Furthermore, the Ni—Ti alloys are poor in weldability and adhesion, posing problems when combined with other materials.